The present invention relates to a semiconductor device. Particularly it relates to a current-limited semiconductor device connected to a load.
An example of a current-limited semiconductor device is an internal combustion engine ignition device (igniter). The internal combustion engine ignition device is a device, for example, that is provided for controlling a gasoline engine used in a car or the like, which is provided with a spark plug for starting combustion by igniting an air-fuel mixture such as an air-gasoline mixture imported into a combustion chamber. FIG. 18 is a conceptual view showing an example of the internal combustion engine ignition device. As shown in FIG. 18, the internal combustion engine ignition device includes primary and secondary coils 201 and 202 provided as ignition coils, a switching unit 203 which turns on and off a low-voltage current flowing in the primary coil 201, a battery 204 which supplies electric power to the ignition coils, and a spark plug 205 which ignites the air-fuel mixture by discharging a high-voltage current induced in the secondary coil 202 in accordance with the turning-on/off of the low-voltage current flowing in the primary coil 201. The switching unit 203 has an amplification function for amplifying a feeble electric signal, and a switching function. Although a bipolar transistor has been used for achievement of the amplification function and the switching function, the bipolar transistor has been replaced by an IGBT (Insulated Gate Bipolar Transistor) in recent years.
A device has been proposed as an internal combustion engine ignition device using an IGBT. That is, an internal combustion engine ignition semiconductor device has: a DC power supply and a switching unit connected to a primary winding of an ignition coil; and a spark plug connected to one end of a secondary winding of the ignition coil, in which a high voltage generated in the secondary winding in accordance with change of a primary current in the ignition coil based on the switching-on/off operation of the switching unit is supplied to the spark plug. The switching unit is an MOS gate structure transistor. In order to limit the coil current of the primary winding to a certain predetermined value, the switching unit at least has a coil current detecting portion, and a circuit for dropping a gate voltage of the MOS gate structure transistor. The internal combustion engine ignition semiconductor device has a current supply circuit for applying a voltage induced by a current flowing into a gate terminal from a main terminal of the MOS gate structure transistor to the gate terminal when the voltage of the main terminal on a high voltage side is higher than the voltage of the gate terminal. In the internal combustion engine ignition semiconductor device, the current supply circuit is composed of at least a plurality of constant current elements connected in series. On this occasion, each of the constant current elements is a depression type IGBT or a depression type MOSFET (MOS gate field-effect transistor) (e.g. see JP-A-2000-310173).
Another device has been proposed as an internal combustion engine ignition device. That is, an ignition semiconductor device is equipped with a switching device which is connected to an ignition coil in series and provided for controlling switching-on/off of a current flowing in the ignition coil, a current limiting circuit which controls the switching device to limit the current flowing in the ignition coil, and a voltage limiting circuit which clamps a voltage released from the ignition coil. The ignition semiconductor device further has a timer circuit which starts its operation in response to an input signal applied to a drive terminal of the switching device and outputs an output signal after elapse of a predetermined time since application of the input signal, and a main current progressive reduction circuit which reduces the current flowing in the switching device in response to the output signal of the timer circuit regardless of continuous application of the input signal. An IGBT is used as the switching device which is an output-stage element in the ignition semiconductor device (e.g. see JP-A-2002-004991).
Generally, both low on-voltage characteristic and low switching loss characteristic are required of an IGBT, for example, used in an inverter for a switching purpose. Although low switching loss characteristic is also required of an IGBT used in an internal combustion engine ignition device, the importance of the low switching loss characteristic of the internal combustion engine ignition device is lower than that of the inverter. This is because the switching operation time of the internal combustion engine ignition device, for example, expressed in terms of a switching time of about 10 μs and a switching frequency of 1 kHz is lower than that of the inverter or the like so that the necessity of considering switching loss is low. For this reason, in the internal combustion engine ignition device, importance can be placed on on-voltage reduction with respect to low on-voltage characteristic and low switching loss characteristic which have a trade-off relation. When a low on-voltage is achieved, the steady loss of the internal combustion engine ignition device can be reduced.
A current flowing in the primary side is limited by a total resistance value of on-resistance of the IGBT and resistance of the ignition coil connected in series. If the on-resistance of the IGBT can be reduced, the degree of freedom for adjusting an inductance value increases because a margin can be given to the resistance of the ignition coil. The increase in the degree of freedom for adjusting the inductance value permits the ignition coil to be easily designed.
The following device has been proposed as an internal combustion engine ignition device in which an on-voltage is reduced. That is, the device has a semiconductor switching element, an overcurrent protecting circuit, and an overcurrent limiting circuit. The semiconductor switching element has a gate terminal, and first and second terminals, in which a main current flows between the first and second terminals when a voltage is applied to the gate terminal. The overcurrent protecting circuit reduces the main current at a first gradient and then reduces the main current at a second gradient steeper than the first gradient when the main current becomes an overcurrent exceeding a predetermined current for a predetermined time or longer. The overcurrent limiting circuit reduces the voltage of the gate terminal instantaneously when the main current becomes an overcurrent further larger than the overcurrent for a time shorter than the predetermined time (e.g. see JP-A-2001-345688).
In the internal combustion engine ignition device, it is however necessary to ensure tolerance to breakdown due to a high-voltage current returned from the secondary coil 202 to the primary coil 201 when the spark plug 205 (see FIG. 18) fails in electric discharge. Because the semiconductor device needs to have a predetermined volume in order to ensure the tolerance of the semiconductor device, it is impossible to make the size of the semiconductor element smaller than a certain predetermined size. For this reason, achievement of low on-resistance cannot lead to cost reduction based on reduction of the size of the semiconductor device.
The internal combustion engine ignition device is provided with a current limiting function which is one function for preventing breakdown. The provision of the current limiting function prevents the coil from being burned out or cut off by an overcurrent and prevents the semiconductor element from being broken down by temperature rise. FIG. 19 is a circuit diagram showing an example of an internal combustion engine ignition device provided with a current limiting function. As shown in FIG. 19, the internal combustion engine ignition device has a main IGBT 103 which makes a low-voltage current flow into a primary coil (see FIG. 18), and a current limiting circuit 107 which controls a current flowing in the main IGBT 103. Protection Zener diodes 104 to 106 are connected.
The current limiting circuit 107 has a sense IGBT 111 which is provided on the side of an external collector terminal 102 and which monitors a current flowing in the main IGBT 103. The current limiting circuit 107 has a sense resistor 112 which is connected between an emitter of the sense IGBT 111 and the ground and which monitors a sense voltage at a node 113 near the emitter of the sense IGBT 111. In addition, the current limiting circuit 107 has a comparator 114, and an MOSFET 116. The comparator 114 detects the fact that the sense voltage at the node 113 reaches a predetermined voltage value. The MOSFET 116 controls the opening/closing state of a gate of the main IGBT 103 in accordance with a result of the detection by the comparator 114. A Vref circuit 115 which sets a voltage value as a reference sense voltage value in advance is connected to the comparator 114. The comparator 114 detects the fact that the sense voltage at the node 113 reaches the voltage value set in the Vref circuit 115. The MOSFET 116 turns off a gate signal of an external gate terminal 101 to thereby limit a current value in the main circuit to a desired range.
Further, since a limiting current value varies according to variation in electric characteristic of a semiconductor element in a real manufacturing process, for example, a trimming circuit or the like is provided. The trimming circuit adjusts the value of the current flowing in the semiconductor element to a desired limiting current value range. For example, there is used a method of making adjustment by trimming the resistance value of the sensor resistor 112 and the reference voltage of the Vref circuit 115.
The following device has been proposed as an internal combustion engine ignition device having a current limiting function. That is, an internal combustion engine ignition device includes an ignition coil, and a switching circuit. The ignition coil has a primary coil, and a secondary coil. The switching circuit breaks a current in the primary coil of the ignition coil based on an ignition signal voltage to thereby generate a high voltage in the secondary coil of the ignition coil for ignition. The ignition signal voltage is a pulse voltage including a leading edge and a trailing edge. The switching circuit does not have any power supply terminal but has an output terminal connected to the primary coil of the ignition coil, an input terminal receiving the ignition signal voltage, and a reference potential terminal. The switching circuit has a switching element, a drive resistor, and a current supply circuit. The switching element is connected between the output terminal and the reference potential terminal so that a current is made to flow into the primary coil of the ignition coil when the switching element is turned on, and the current in the primary coil is broken when the switching element is turned off. The drive resistor is provided opposite to the switching element. The current supply circuit is connected between the input terminal and the reference potential terminal so that the current supply circuit supplies a drive current to the drive resistor. The current supply circuit starts supply of the drive current at the leading edge based on the ignition signal voltage to thereby turn on the switching element, and the current supply circuit breaks the drive current at the trailing edge to thereby turn off the switching element. The current supply circuit further includes a constant current circuit which keeps the drive current constant and supplies the constant drive current to the drive resistor (e.g. see Japanese Patent No. 3842259).
The following device has been proposed as another internal combustion engine ignition device having a current limiting function. The internal combustion engine ignition device includes an ignition coil, an ignition switching unit, and a spark plug. The ignition coil has a primary winding, and a secondary winding. When a primary current flowing in the primary winding is broken, a high voltage for ignition is generated in the secondary winding. The ignition switching unit makes/breaks the primary current flowing into the primary winding of the ignition coil. The spark plug is connected to the secondary winding so that spark discharge is generated when the high voltage for ignition is applied to the spark plug.
The internal combustion engine ignition device further includes a primary current limiting unit which limits a increase rate of the primary current for a predetermined period since the start time of current conduction in the primary coil so that false ignition can be prevented from being caused in the internal combustion engine by spark discharge generated in the spark plug in a different time from the ignition time by an induced voltage reversed in polarity to the ignition high voltage generated in the second coil in accordance with the current conduction in the primary coil (e.g. see JP-A-2003-214307).
In the aforementioned current limiting circuit 107 (see FIG. 19), however, there is a feedback loop which controls a gate signal to keep a current value at a constant value after the current flowing in the main IGBT 103 increases and reaches a desired current value. FIG. 20 is a characteristic graph showing the waveforms of a gate signal and a collector current at the time of current limiting operation. As shown in FIG. 20, when the gate signal is turned on at a first time t1, the collector current Ic begins to increase based on an induced load (L load). When the collector current Ic reaches a desired current value Icontrol at a second time t2, the current limiting circuit 107 starts to operate. On this occasion, there is a time lag Δt before control of the gate signal starts to work. The collector current Ic increases continuously during the time lag Δt. For this reason, the collector current Ic exceeds the desired current value Icontrol (hereinafter referred to as current overshoot) at a third time t3 when control of the gate signal works. When the gate signal is controlled, the collector current Ic decreases suddenly to the desired current value Icontrol from a maximum current value Ipeak increased by the current overshoot. The current waveform is apt to be vibrated (like the current waveform designated by a double-dash chain line in FIG. 20) by the feedback loop then performed continuously for keeping the current value at the desired current value Icontrol.
This phenomenon arises from deviation of the limiting current value due to the difference in bias condition between the main IGBT 103 and the sense IGBT 111 because the sense resistor 112 is connected to the emitter side of the sense IGBT 111. It is hence very difficult to keep balance in the control circuit. In addition, since the current control circuit has a complicated structure and requires the sense IGBT, the control circuit is large-sized. For this reason, there occurs a problem that the total size of the semiconductor device is large to result in cost increase. Particularly in the internal combustion engine ignition device, the ratio of the control circuit to the semiconductor device as a whole becomes large because the area of the main IGBT per se is about several mm2.
In the internal combustion engine ignition device, the main IGBT 103 and the control circuit including the current limiting circuit 107, etc. can be formed integrally on one semiconductor substrate. On this occasion, a gate signal is generated by use of a horizontal type n-channel MOSFET and a depression type (normally-on type) MOSFET. The depression type MOSFET is turned on when a gate voltage is not applied thereto, but the depression type MOSFET is turned off at the time of operation. A power voltage for these MOSFETs may be supplied from another external power supply or may be supplied as an external gate signal. When the power voltage for these MOSFETs is supplied from another external power supply, the power supply voltage varies little. However, when the power voltage is supplied as an external gate signal, the voltage value of the gate signal varies widely according to a load state, etc. imposed on the circuit.